1. Field of the Invention
This invention relates to a recording-reproducing device for a magnetic disk used as a recordable terminal device of computer, word processor and the like.
2. Description of the Prior Art
Hitherto, the conventional intermittent feed mechanism for shifting a carriage having a magnetic head mounted thereon in the radial direction of a magnetic disk had employed the so-called steel belt system in many cases, but this steel belt system had the drawbacks that setting of the belt and adjusting of its tension are troublesome and the factor of space utilization is bad. Recently, instead of the above, a carriage shifting mechanism based on the screw shaft system is going to be employed in the disk recording-reproducing device, for such a reason that it is suitable for decreasing a device size.
FIG. 9 is a plane view illustrating the conventional carriage shifting mechanism of the screw shaft system as noted above. In this drawing, 1 is a stepping motor mounted on a chassis 2, and 3 is a screw shaft formed on its outer periphery with a spiral groove 3a, which passes through a permanent magnet (not shown) of a cylinder shape, that is a rotor of the stepping motor 1, and is locked thereto, one end of the screw shaft being supported by a bearing (not shown) of a casing of the stepping motor 1 and the other end being supported by a bearing 4. 5 is a carriage with a read/write magnetic head 6 mounted thereon, which is shifted along two guide shafts 7, 8. 9 is an engaging element attached to the carriage 5 whose point 9a of a cone shape is engaged with the spiral groove 3a of the screw shaft 3.
In the foregoing structure, as the screw shaft 3 is rotated in the forward/reverse directions by the stepping motor 1, the point 9a fitted in the spiral groove 3a moves in the arrow directions shown in the drawing and, as a result, the carriage 5 is shifted reciprocatingly along the guide shafts 7, 8 a distance corresponding to the degree of rotation of the screw shaft 3.
Specifically, the spiral groove 3a of the screw shaft 3 in the foregoing structure has the same angle of lead .theta. over the whole periphery and exhibits a straight line when developed, as illustrated in FIG. 10. Accordingly, if the system is designed so that movement of the carriage 5 is stopped intermittently at eight points, for example, corresponding to the turning angles 0.degree., 45.degree., 90.degree., 135.degree., 180.degree., 225.degree., 270.degree., 315.degree. of the screw shaft 3, the carriage 5 shows a minute discrepancy in its positioning as the results of inertia of the carriage 5, influence of damped oscillation of, variation in rotation control of the stepping motor 1, and the like. Thus, the conventional device had the drawback that some difficulty is easy to occur in performing accurate read/write operation of the magnetic head 6 in relation to a magnetic track on the disk.